A monopoly produces widgets at a marginal cost of $10 per unit and zero fixed costs. It faces an inverse demand function given by P = 50 - Q. Which of the following is the marginal revenue function for the firm?
A. MR = 60 - 2Q
B. MR = 50 - Q
C. MR = 100 - Q
D. MR = 50 - 2Q